1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to a vertically-aligned (VA) LCD having a color filter (CF) substrate with depressions for improving the viewing angle performance thereof.
2. Description of the Related Art
Vertically-aligned (VA) LCD is a mode using a negative liquid crystal material and vertical alignment film. When no voltage is applied, liquid crystal molecules are aligned in a vertical direction and black display appears. When a predetermined voltage is applied, the liquid crystal molecules are aligned in a horizontal direction and white display appears. Comparing to the twisted nematic (TN) LCD, the vertically-aligned (VA) LCD provides higher contrast, higher response speed, and excellent viewing angle characteristic for white display and black display. However, the VA LCD still has a critical drawback of a narrow viewing angle range. This poses a problem that the application of the VA LCD is limited.
It is known that viewing angle performance of a VA LCD can be improved by setting the orientation of the liquid crystal molecules inside pixels to a plurality of mutually different directions. European Patent Publication No. 0884626-A2 discloses a VA LCD having domain regulating means for regulating the orientation of a liquid crystal in which liquid crystal molecules are aligned obliquely when a voltage is applied so that the orientation will include a plurality of directions within each pixel.
In FIGS. 1a to 1c, as the domain regulating means, pixel electrodes 12 on a TFT (Thin Film Transistor) substrate are slitted, and the common electrode 13 on a CF (color filter) substrate is provided with protrusions 20. As shown in FIG. 1a, in a state in which no voltage is applied, liquid crystal molecules are aligned vertically to the surfaces of the substrates. When an intermediate voltage is applied, as shown in FIG. 1b, electric fields oblique to the surfaces of the substrates are produced near the slits of the electrodes (edges of the electrodes). Moreover, liquid crystal molecules near the protrusions 20 slightly tilt relative to their state while no voltage is applied. The inclined surfaces of the protrusions 20 and the oblique electric fields generated between the substrates determine the directions in which the liquid crystal molecules are tilted. The orientation of the liquid crystal is divided into different directions along a plane defined by each pair of protrusions 20 and the center of each slit thereby improving viewing angle performance. FIG. 2 shows another example of realizing the domain regulating means by providing protrusions 20 alternately on the electrodes 12 and 13, and coated with the vertical alignment films 22. When no voltage is applied, the liquid crystal molecules are aligned vertically to the surfaces of the substrates in principles. On the inclined surfaces of the protrusions, however, the liquid crystal molecules are aligned at a little tilt. When a voltage is applied as shown in FIG. 2, the liquid crystal molecules are tilted in different directions.
It is known that protrusions or electrode slits can be formed on the TFT substrate by utilizing the conventional process as it is. However, it is very difficult to form protrusions or electrode slits on CF substrate without introducing any additional step. Therefore, it is proposed that the domain regulating means is realized by using a depression instead of a protrusion (see FIG. 3 and FIG. 4). As shown, color filters 24 having depressions 22 are formed on the CF substrate 2. An electrode 12 is formed on the color filters 24; hence, areas on the electrode 12 at which corresponds to the depressions 22 are recessed. Protrusions 20 on the electrodes 13 are formed on the TFT substrate 4. Vertical alignment films (not shown) are further formed on the electrodes 12 and 13, respectively. As shown in FIG. 3, in a state in which no voltage is applied, liquid crystal molecules are aligned vertically to the surfaces of the substrates. When an intermediate voltage is applied, oblique electric fields (indicated by dotted line shown in FIG. 4) are generated near the depressed portions of the electrode 12 and the protrusions 20. When the color filters 24 are formed on the CF substrate 2, it is easy to form the depressions 22 at the same time without introducing any additional step. However, for liquid crystal molecules near the depressed portions of the electrode 12, the tilt direction of the liquid crystal molecules under electric field (see FIG. 4) is opposite to the directions in which the liquid crystal molecules are pre-tilted because of the depressed portions of the electrode 12 (see FIG. 3). This leads to a problem that its response (switching) speed is not fast enough; particularly, a response speed for transition from a state in which no voltage is applied to a state in which a voltage is applied is relatively slow.
The present invention therefore seeks to provide a VA LCD that overcomes, or at least reduces the above-mentioned problems of the prior art.
It is a primary object of the present invention to provide a vertically-aligned (VA) LCD having a color filter (CF) substrate with filled depressions that substantially obviates one or more problems due to limitations and disadvantages of the prior art.
To achieve the object described above, the present invention provides a liquid crystal display device comprising: a first substrate and a second substrate processed for vertical alignment; a liquid crystal having a negative dielectric constant anisotropy and being sandwiched between the first and second substrates; a plurality of color filters on the first substrate, each of the color filters having at least a first depression formed therein; a common electrode on the color filters, the common electrode having a plurality of second depressions corresponding to the first depressions of the color filters; and a dielectric material filled into the second depressions of the common electrode, the dielectric material having a dielectric constant smaller than the dielectric constant of the liquid crystal.
According to a first embodiment, the second depressions of the common electrode are filled by the dielectric material thereby forming a substantially flat surface level with the surface of the first substrate. Since the liquid crystal molecules are aligned vertically to the surface of the substrate in principle, the pre-tilt angle of liquid crystal molecules near the second depressions of the common electrode is minimized thereby improving the response (switching) speed for transition from a state in which no voltage is applied to a state in which a voltage is applied.
According to a second embodiment, the second depressions of the common electrode are filled by the dielectric material thereby forming protrusions corresponding to the second depressions. Therefore, for liquid crystal molecules near the second depressions of the common electrode, the tilt direction of the liquid crystal molecules under electric field matches the directions in which the liquid crystal molecules are pre-tilted because of the protrusions formed corresponding to the second depressions.